Answer:
 848.3 yards
Step-by-step explanation:
If Joel were to draw a diagram of his zipline, he would see that it is the hypotenuse of a right triangle with a leg of length 304 yards opposite the angle of 21 degrees. The sine relation comes into play when these values are given:
 Sin = Opposite/Hypotenuse
The hypotenuse can be found as ...
 Hypotenuse = Opposite/Sin
 zipline length = (304 yd)/sin(21°)
 zipline length ≈ 848.3 yd
